Don't try to add an existing directory

[Neil Martinsen-Burrell]

Traceback (most recent call last):
      File "./gen_random_history.py", line 166, in <module>
        main()
    Traceback (most recent call last):
      File "./gen_random_history.py", line 166, in <module>
        main()
      File "./gen_random_history.py", line 162, in main
        options.config, options.show_config, options.init_dir) File "./gen_random_history.py", line 133, in generate
        b, t = transition.evolve(revisions, mutators, editors)
      File "/Users/nmb/src/bzr/mcrepogen/mcrepogen/transition.py", line 57, in
evolve tree = transition.step(tree)
      File "/Users/nmb/src/bzr/mcrepogen/mcrepogen/transition.py", line 79, in
step
        tree = mutator.mutate(tree) File "/Users/nmb/src/bzr/mcrepogen/mcrepogen/mutators.py", line 179, in m
utate
        input_tree.add_directory(new_path)
      File "/Users/nmb/src/bzr/mcrepogen/mcrepogen/bzrtreebranch.py", line 190, in add_directory
        os.mkdir(absolute_directory)
    OSError: [Errno 17] File exists: '/private/var/folders/o1/o1uqBGiCHxysyrhc-
DXvGE+++TI/-Tmp-/tmpM5KinK/sopping/jawbone/thoughtlessly'

Fix this by trapping OSError and checking for errno.EEXISTS